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2016-03-20T22:06:58+05:30

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Let the number of boys be 'x'
and girls in the school be 'y'.
therefore x + y = 60
since mean = sum of observations by no of observation

50*x + 30*y / x + y = 40
 50x + 30y = 40*60
50x + 30y = 2400
5x + 3y = 2400/10 = 240

so we have 5x + 3y = 240 and x + y = 60
multiplying second equation by 3 , we get ,
5x + 3y =240
3x + 3y = 180
by subtracting , y terms will be cancelled , and 
2x = 60 
x = 30

substituting x in 2nd equation , we get,

30 + y = 60
y =  60 - 30 = 30
hence x = 30 and y = 30
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